1. Field of the Invention
The invention relates to a method of steepening the edges of a color signal, in which method a control signal is generated which is obtained by differentiation, rectification and subsequent differentiation of the rectified first differentiation, and is temporarily assigned to the color signal, and in which method, in dependence upon the control signal, chrominance values of reference pixels of the color signal are copied on edge pixels of the color signal in such a way that the chrominance values of reference pixels preceding the edge pixels are copied on the edge pixels in the case of a positive control signal, and the chrominance values of reference pixels following the edge pixels are copied on the edge pixels in the case of a negative control signal.
2. Description of the Related Art
Such an arrangement is known from the Philips ICs SAA4970 and SAA7158. In these ICs, the edges of a color signal or two color difference signals in this color signal are steepened by means of two-fold differentiation. The result of the second differentiation is used to copy the chrominance values of pixels preceding an edge, on pixels in the region of the edge in the case of a positive signal, and to copy color pixel values following the edge on pixels in the region of the edge in the case of a negative signal. This leads to steeper edges. Moreover, a "Hill" protection is performed, serving the purpose of minimally distorting very narrow pulses. This Hill protection ensures that, at zeros of the first differentiation, the copying process is not performed beyond the zero. In spite of this Hill protection, the problem occurs in this arrangement that, for example, for neighboring pulses having different chrominance values, these values may be mixed, due to the above-described copying process, in the transition region between the two pulses. For example, color pixel values of a first red pulse are transmitted from its final region to the initial region of a subsequent green pulse, and conversely. The colors are then mixed in a clearly visible way, or colors appear in a region which is actually colorless.